Distributed semi-autonomous phased arrays for subsurface VLF transmission

ABSTRACT

A system for subsurface transmission includes an array of very low frequency (VLF) transmitter nodes supported by semi-autonomous maritime, airborne, or space platforms spaced at regular intervals from their nearest neighbors and phased to localize VLF coverage to some desired area on a body of water.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to and is a divisional application of U.S.patent application Ser. No. 16/868,443 filed on May 6, 2020, whichrelates to, claims the benefit of, and claims priority to U.S.Provisional Patent Application 62/871,900, filed on Jul. 9, 2019, whichare incorporated herein by reference as though set forth in full. Thisapplication is related to U.S. Provisional Patent Application Ser. No.62/872,045 filed on Jul. 9, 2019 and entitled “An Array of VLFScatterers for Control of Electromagnetic Wave Propagation on the OceanSurface” the disclosure of which is hereby incorporated herein byreference.

STATEMENT REGARDING FEDERAL FUNDING

This invention was made under U.S. Government Contract N66001-19-C-4018.The U.S. Government may have certain rights in this invention.

TECHNICAL FIELD

This disclosure relates to very low frequency (VLF) transmitters andantennas.

BACKGROUND

Very low frequency or VLF is the International Telecommunications Union(ITU) designation for radio frequencies (RF) in the range of about 3 to30 kilohertz (kHz), corresponding to wavelengths from about 100 to 10kilometers, respectively. The VLF band is used for a few radionavigation services, government time radio stations (broadcasting timesignals to set radio clocks) and for secure military communication.Since VLF waves can penetrate a useful distance into saltwater, they areused for military communication with underwater platforms. Herein, VLFmay refer to radio frequencies (RF) in the range of 3 to 30 kilohertz(kHz), or other very low frequencies.

Prior art very low frequency (VLF) transmitters have been used forcommand and control of submerged platforms, and are typically largestationary monolithic structures. These prior art VLF transmitters aremassive in size and require a large land area, and in addition have highoperational costs.

A variety of other VLF transmitter architectures have been proposed andinvestigated. The most common type of VLF transmitter is a large groundbased station, such as the Cutler station in Maine. Typically these VLFtransmitters are constructed of one or more very large top-loadedmonopole structures designed to couple energy into the earth-ionospherewaveguide (EIW) to provide VLF coverage over large sections of theearth.

Another prior art VLF transmitter architecture utilizes a long wireantenna trailing behind an airplane to achieve VLF transmission from asingle mobile platform. Such an architecture is described in U.S. Pat.No. 4,335,469, issued Jun. 15, 1982, which is incorporated herein byreference.

Yet another prior art VLF transmitter architecture employs aerostats andconsists of a ground based VLF source feeding a long conductor, which issupported by a lighter than air structure, such as an aerostat or aballoon. For example, U.S. Pat. No. 4,476,576, issued Oct. 9, 1984, andU.S. Pat. No. 4,903,036, issued Feb. 20, 1990, each describe prior artVLF communication systems, which utilize a cable connected to a deployedaerostat, which acts a tether and a VLF antenna. The cable is connectedto an RF transmitter located on the ground. In U.S. Pat. No. 4,903,036,issued Feb. 20, 1990, the tether is set to have a length that is roughlyone quarter of the desired electromagnetic wavelength.

Another prior art VLF transmitter architecture strings a long conductorbetween two satellites to enable VLF/ELF (very low frequency/extremelylow frequency) transmission from orbit, such as the NASA tetheredsatellite system (TSS).

While all of these prior systems are effective at generating VLFradiation, they each have relied on massive physical size to achieveefficient operation. These systems also do not provide a method ofcontrolling VLF signal coverage.

Other techniques have also been proposed in the literature for VLFgeneration such as high frequency (HF) heating of the ionosphere but aredependent on conditions in the ionosphere, require massive HFtransmitters, and provide limited ability to manage VLF coverage.

What is needed is an improved VLF transmitter antenna system and amethod of phasing and coordinating arrays of maritime, airborne orspace-borne VLF transmitters to create a localized region of VLFcoverage at and below the surface of a body of water. Also needed is away of controlling the coverage of a transmitted VLF signal. Theembodiments of the present disclosure answer these and other needs.

SUMMARY

In an embodiment disclosed herein, a system for subsurface transmissioncomprises an array of very low frequency (VLF) transmitter nodes hostedon maritime platforms arranged in an array on a body of water, each VLFtransmitter node comprising: a controller; a VLF transmitter coupled tothe controller; a data buffer coupled to the controller; a navigationsubsystem coupled to the controller; and a communications transceivercoupled to the controller; wherein each VLF transmitter node ispositioned less than one half wavelength of a desired very low frequencyfrom another VLF transmitter node; wherein the array of VLF transmitternodes has a center location; wherein data to be transmitted is receivedby the communications transceiver in each VLF transmitter node andstored in the data buffer in each VLF transmitter node; and wherein eachrespective VLF transmitter node transmits the data in the data bufferwith phasing or timing based on position data from the navigation sensorin the respective VLF transmitter node and the position data relative tothe center location.

In another embodiment disclosed herein, a system for subsurfacetransmission comprises an array of very low frequency (VLF) transmitternodes each hosted on an airborne platform and arranged in a line orplane normal to a surface of a body of water, each VLF transmitter nodecomprising: a controller; a VLF transmitter coupled to the controller; adata buffer coupled to the controller; a navigation subsystem coupled tothe controller; and a communications transceiver coupled to thecontroller; wherein each VLF transmitter node is positioned less thanone half wavelength of a desired very low frequency from another VLFtransmitter node; wherein data to be transmitted is received by thecommunications transceiver in each VLF transmitter node and stored inthe data buffer in each VLF transmitter node; and wherein eachrespective VLF transmitter node transmits the data in the data buffer ofthe respective VLF transmitter node with phasing or timing to enhanceradiation in a direction towards the body of water and to suppressradiation in other directions.

In yet another embodiment disclosed herein, a system for subsurfacetransmission comprises a first array of very low frequency (VLF)transmitter nodes each hosted on an airborne platform and arrangedsubstantially parallel to a surface of a body of water, each VLFtransmitter node comprising: a controller; a VLF transmitter coupled tothe controller; a data buffer coupled to the controller; a navigationsubsystem coupled to the controller; and a communications transceivercoupled to the controller; wherein each VLF transmitter node ispositioned less than one wavelength of a desired very low frequency fromanother VLF transmitter node; wherein data to be transmitted is receivedby the communications transceiver in each VLF transmitter node andstored in the data buffer in each VLF transmitter node; and wherein eachrespective VLF transmitter node transmits the data in the data buffer ofthe respective VLF transmitter node with phasing or timing to enhanceradiation in a direction towards the body of water and to suppressradiation in other directions.

In still another embodiment disclosed herein, a system for subsurfacetransmission comprises an array of satellite nodes arranged in a planesubstantially parallel to a surface of a body of water, each satellitenode comprising: a pair of satellites, each pair of satellitescomprising: a tether between the pair of satellites; a dipole antennaplaced along the tether and oriented parallel to the body of water; acontroller; a VLF transmitter coupled to the dipole antenna and to thecontroller; a data buffer coupled to the controller; a navigationsubsystem coupled to the controller; and a communications transceivercoupled to the controller; wherein each satellite node is positionedless than one wavelength of a desired very low frequency from anothersatellite node; wherein data to be transmitted is received by thecommunications transceiver in each satellite node and stored in the databuffer in each satellite node; wherein each respective satellite nodetransmits the data in the data buffer of the respective node withphasing or timing to enhance radiation in a direction towards the bodyof water and to suppress radiation in other directions; and wherein allthe satellite nodes begin transmission of data in the data buffer atsubstantially the same time with substantially the same phase.

These and other features and advantages will become further apparentfrom the detailed description and accompanying figures that follow. Inthe figures and description, numerals indicate the various features,like numerals referring to like features throughout both the drawingsand the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram showing major subsystems within a VLFtransmitter node in accordance with the present disclosure;

FIG. 2 shows a high level schematic of a VLF transmitter showing anantenna, a matching network, and an RF driver in accordance with thepresent disclosure;

FIG. 3 shows an array of VLF transmitters spaced less than one halfwavelength from their nearest neighbors and phased to radiate maximallyin a direction substantially parallel to the ocean surface and toradiate minimally in a direction to the ionosphere in accordance withthe present disclosure;

FIG. 4 shows an array of VLF transmitters that generates a slow wavealong the surface and is phased to suppress radiation in any otherdirection in accordance with the present disclosure;

FIG. 5 shows an array of VLF transmitters, each supported by an aerialvehicle, arranged in a line normal to the ocean surface and coordinatedto create a radiation maximum in the direction of the ocean surface inaccordance with the present disclosure;

FIG. 6 shows a variation on the embodiment in FIG. 5 in which a localVLF sensor is placed on the surface of the ocean near a desired centerof the coverage area to measure an amplitude of the VLF field generatedby the array of VLF transmitters to provide feedback to improve thecoordination and phasing of the array of VLF transmitters in accordancewith the present disclosure;

FIG. 7 shows an array of VLF transmitters arranged in a planesubstantially parallel to the ocean surface and supported by an aerialvehicles and phased to radiate maximally in a direction substantiallynormal to the ocean surface and to radiate minimally in a directionparallel to the ocean surface in accordance with the present disclosure;

FIG. 8 shows a variation on the embodiment in FIG. 7 in which a localVLF sensor is placed on the surface of the ocean near a desired centerof the coverage area to measure an amplitude of the VLF field generatedby the array of VLF transmitters to provide feedback to improve thecoordination and phasing of the array of VLF transmitters in accordancewith the present disclosure;

FIG. 9 shows an array of VLF transmitters arranged in a planesubstantially parallel to the ocean surface and supported by satellitesand phased to radiate maximally in a direction substantially normal tothe ocean surface and to radiate minimally in a direction parallel tothe ocean surface in accordance with the present disclosure; and

FIG. 10 shows a variation on the embodiment in FIG. 9 in which a localVLF sensor is place on the surface of the ocean near a desired center ofthe coverage area to measure an amplitude of the VLF field generated bythe array of VLF transmitters to provide feedback to improve thecoordination and phasing of the array of VLF transmitters in accordancewith the present disclosure.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toclearly describe various specific embodiments disclosed herein. Oneskilled in the art, however, will understand that the presently claimedinvention may be practiced without all of the specific details discussedbelow. In other instances, well known features have not been describedso as not to obscure the invention.

Conventional VLF transmitters used for command and control of submergedplatforms are typically large monolithic structures, requiring massivesize and operational costs to achieve their mission. These systemstypically rely on propagation off the ionosphere and consequentlybroadcast signals over extremely large areas, making transmit signalsrelatively easy to intercept.

The disclosed invention utilizes arrays of compact VLF transmittersconfigured to enhance radiation in the direction of the ocean surfaceand to suppress radiation outside of a desired coverage area to suppresssignal reception and interception. This approach allows VLF coverage tobe localized to a desired portion of the ocean and reduces the neededtransmitter power and cost associated with operating a VLF transmitter.The arrays of VLF transmitter nodes 10 may be on semi-autonomousplatforms and phased to create an array which localizes VLF coverage tosome portion of the ocean surface. The semi-autonomous platforms may bemaritime, airborne, or space platforms spaced at regular intervals fromtheir nearest neighbors.

In this disclosure, the ocean is an example of a body of water. Ratherthan being an ocean, the body of water may be any body of water, such asa lake.

In radio frequency (RF) engineering, it is common to use linear andcollinear antenna arrays; however, at VLF frequencies conventionalfeeding networks used to supply and drive these arrays are impracticaldue to the spacing between VLF transmitter nodes in the array and thevery long wavelength. The present invention describes a system andmethod of phasing and coordinating the arrays of maritime, airborne orspace-borne VLF transmitter nodes.

Now referring to FIG. 1 , a block diagram of one VLF transmitter node 10is shown. As shown in FIG. 1 , the VLF transmitter node 10 may include aVLF transmitter 12, a controller 14, a data buffer 16, a clock 18, anavigation subsystem 20 and a communications transceiver 22. Thecontroller may include a processor, such as a microprocessor or acomputer. The navigation subsystem may be a global positioning system(GPS), and include an inertial sensor, such as a gyroscope and so on.The communications transceiver 22 is configured to operate in a banddifferent than VLF, and may be a high frequency (HF), very highfrequency (VHF), ultra high frequency (UHF), or satellite transceiver orradio.

The VLF transmitter 12, as shown in FIG. 2 , may include an antenna 24,an RF driver 26, and a matching network 28. The antenna 24 may be anelectrically small monopole, a dipole or a loop antenna. The matchingnetwork 28 may include inductors, capacitors, resistors, switches,and/or any combination of such components appropriate to resonate theantenna and improve transmitter efficiency over the desired VLFbandwidth. The RF driver for the transmitter may include an RF signalgenerator 30 electrically connected to an amplifier 32.

The electromagnetic fields generated by the array of VLF transmitternodes 10 may be controlled by varying the relative phase transmitted byeach VLF transmitter node 10 in the array. Coordination and phasing ofeach VLF transmitter node 10 in the array with its neighbors may beachieved through a variety of techniques. The data to be transmitted maybe received via communications transceiver 22 and stored in the databuffer 16 of each VLF transmitter node 10 for transmission at a timewhich may be different for each respective VLF transmitter node 10 inorder to properly phase the combined transmission of the array of VLFtransmitter nodes 10.

In one embodiment in order to properly phase the transmission from eachVLF transmitter node 10 in an array, a respective VLF transmitter node10 begins transmitting the data from its data buffer 16 at a time and ina radiation mode predetermined by commands received by the respectiveVLF transmitter node 10 via the communications transceiver 22. Inanother embodiment, each respective VLF transmitter node 10 autonomouslydetermines the time and radiation mode to begin transmission of the datain the data buffer 16. The time of transmission and radiation mode of arespective VLF transmitter node 10 is relative to other nodes 10 in thearray and is relative to the position of the respective VLF transmitternode 10 in the array of nodes and the radiation mode.

A VLF transmitter node 10 in the array may operate autonomously to senseits position relative to other VLF transmitter nodes 10 in the array andto control its position in the array using on-board autonomy algorithmsprocessed in the controller and some form of propulsion on the platformhosting the VLF transmitter node 10. In this embodiment, the mission andposition for each VLF transmitter node 10 in the array may bepreprogrammed. Position data may be determined by the navigation system20 through an external source such as GPS, and by using an onboardinertial sensor, such as a gyroscope. Attitude and orientation of eachplatform that has an VLF transmitter node 10 may be maintained bymeasuring relative changes via the onboard inertial sensor and makingcorrections using engines and/or propulsion systems on the platformhosting the VLF transmitter node 10. The relative phasing for eachrespective transmitter is determined based on the estimated position andorientation of the transmitter relative to other transmitters in thearray of nodes.

In another embodiment, one VLF transmitter node 10 in the array isdesignated as a master node and determines the positions and operationsof the other VLF transmitter nodes 10 in the array through on-boardautonomy algorithms or through instructions received by the master nodevia a long-haul communication channel, which may use the communicationstransceiver 22. In this embodiment, communication between the masternode and the other VLF transmitter nodes 10 in the array may be achievedusing a local communication network using the communications transceiver22. The communications transceiver 22 in each VLF transmitter node 10may include the local communication network, and at least the masternode needs to include the long-haul communications channel. Each VLFtransmitter node 10 may include both the local and long-haulcommunications channels to provide redundancy, as well as making themanufacture of each VLF transmitter node 10 the same. The communicationstransceiver 22 in each VLF transmitter node 10 may be a high frequency(HF), very high frequency (VHF), ultra high frequency (UHF), orsatellite transceiver or radio. The master node coordinates the positionand phasing of each VLF transmitter node 10 in the array, and thephasing or transmission time for each VLF transmitter node 10 may bedetermined by timing signals received through an external source such asa GPS reference via the navigation system 20, or through local signalsemitted from the master node to the other VLF transmitter nodes 10 inthe array.

In another embodiment, each node in the array is coordinatedindividually through commands received from a remote operator via thelong-haul communication channel. In this embodiment, position,orientation, phasing, and timing are all received by each VLFtransmitter node 10 through the long-haul communication channel via thecommunications transceiver 22 in each node, which again may be highfrequency (HF), very high frequency (VHF), ultra high frequency (UHF),or satellite transceiver or radio.

As shown in the embodiment of FIG. 3 , an array of electrically smallVLF transmitter nodes 40 may be supported on semi-autonomous maritimeplatforms 41 distributed on the ocean surface, preferably in a regulargrid, and positioned less than one half wavelength from one another. Theregular grid may be a linear 1 dimensional array, or a 2 dimensionalarray. Each maritime platform 41 may be any variety of buoyant vessel,such as a boat, barge, or buoy. In this embodiment, antennas 24 for eachVLF transmitter nodes 40 are polarized parallel to the ocean surface formagnetic antennas, such as loops, or normal to the ocean surface forelectric antennas, such as monopoles and dipoles.

Preferably the VLF transmitter nodes 40 in the array shown in FIG. 3 arespaced roughly a quarter wavelength from their nearest neighbor andphased to enhance radiation in a direction parallel to the oceansurface, while suppressing radiation in other directions. In FIG. 3 anexample linear one dimensional array is shown and the radiation is shownas endfire radiation 38. In one example, each respective VLF transmitternode 40 autonomously knows via navigation sensor 20 and informationreceived via communications transceiver 22 its distance (rho) from adesired center of the array. In FIG. 3 , the master node 42 is shown atthe center of the array; however, the master node 42 does not need to beat the desired center of the array. In another embodiment, eachrespective VLF transmitter node 40 is informed via the communicationstransceiver 22, as described above, of its desired position in the arrayand therefore the distance (rho) from a desired center of the array.

If the master node 42 is at the desired center of the array, then givena time, t0,when the master node 42 begins transmission, each respectiveVLF transmitter node 40 begins transmission with an added delay, so thata respective VLF transmitter node 40 at a distance rho from the masternode 42 at the center of the array begins transmission at time(t0+vg*rho), where vg is a group velocity of a Norton wave, rho is adistance from a desired center of the array, and t0 is a time the masternode 42 at the center of the array begins transmission. The symbol *indicates a multiplication.

The equation above for transmission time from each VLF transmitter nodeis correct, even if master node 42 is not at the center of the array andeven if there is no master node, as long as “t0” is the time at which anactual node or a virtual node at the center of the array beginstransmission.

In the embodiment shown in FIG. 3 , the resulting radiation pattern maybe a doughnut shaped radiation pattern strongly directed toward thehorizon, or parallel to the ocean surface. This radiation pattern isintended to enhance coupling into the Norton surface wave. This may alsosuppress coupling into the skywave via the ionosphere, thereby betterlocalizing VLF signal coverage and more efficiently directing radiatedpower along the ocean surface; however, this effect has not been fullyinvestigated to date.

In a related embodiment, the phase delay for each VLF transmitter node40 may be greater than vg*rho, thereby generating a slow wave. In thisembodiment, transmission for a respective VLF transmitter node 40 atdistance rho begins at time (t0+n*vg*rho), where n is a factor greaterthan 1 corresponding to a synthesized index of refraction for the wave,vg is a group velocity of a Norton wave, rho is a distance from adesired center of the array, and t0 is a time an actual node, which canbe, for example, the master node 42 or a virtual node at the center ofthe array begins transmission. This configuration generates a travelingwave electromagnetic field 43 within the confines of the array, as shownin FIG. 4 , but does not radiate efficiently away from the array. Thisconfiguration also suppresses radiation upward to the ionosphere.

In another embodiment, this invention includes an array of electricallysmall VLF transmitter nodes 44 arranged in a line or plane normal to thesurface of the ocean and supported by airborne vehicles 46, as shown inFIG. 5 . The airborne vehicles may be any variety of fixed wing,rotorcraft, lighter than air vehicles, drones, or unmanned airbornevehicles (UAVs). In this embodiment, antennas 24 for each node 44 can beoriented parallel or normal to the surface of the ocean. VLF transmitternodes 44 in the array are preferably spaced roughly a quarter wavelengthfrom their nearest neighbor and are phased to enhance radiation in thedirection of the ocean surface, while suppressing radiation in otherdirections. The phase of each VLF transmitter node 44 may be chosen tobe delayed in proportion to the propagation phase from the next highestin elevation VLF transmitter node 44. This is preferably implemented asa time delay, where each respective VLF transmitter node 44 beginstransmission at time (t0+vg*(hmax-h)), where h is the height of arespective VLF transmitter node 44, hmax is the height of the top orhighest 45 VLF transmitter node 44 in the array, vg is a group velocityof an electromagnetic wave in air, and t0 is a time the top or highest45 VLF transmitter node 44 in the array begins transmission.

In a variation on this embodiment a VLF sensor 48 is placed on thesurface of the ocean within or near, and preferably at the center of,the coverage area 49, as shown in FIG. 6 . The sensor 48 measures a setof at least one characteristic, for example strength, of the VLF signalgenerated by the array of VLF transmitter nodes 44 at a point within ornear, and preferably at the center of, the coverage area 49. Then thesensor 48 transmits, via a radio frequency signal at a noninterferingand different frequency, the relevant characteristics of the measuredVLF signal. This transmission is then received by each VLF transmitternode 44 by its communication transceiver 22 and used as input toon-board adaptive beamforming algorithms operating in the controller 14to correct the phasing or timing of a transmission of each VLFtransmitter node 44 in the array and may also be used to adjust theposition of the VLF transmitter node 44 in the array.

In another embodiment, this invention includes an array of electricallysmall VLF transmitters 50 arranged in a line or in a plane substantiallyparallel to the surface of the ocean and supported by airborne vehicles52, as shown in FIG. 7 . By substantially parallel it is meant that theVLF transmitters 50 are preferably within ⅛ of the transmissionwavelength from a plane substantially parallel to the surface of theocean, where they are in an actual plane or line that is parallel to thetangent line of the ocean surface, or where the VLF transmitters 50 areall at the same elevation above the earth's surface. These two cases arethe same for small array sizes but slightly diverge for large arraysizes.

The airborne vehicles 52 may be any variety of fixed wing, rotorcraft,lighter than air vehicles, drones or UAVs. In this embodiment, antennasfor each node may be oriented parallel or normal to the surface of theocean. Nodes in the array are spaced roughly one half wavelength (andless than one wavelength) from their nearest neighbor and are phased toenhance radiation in the direction of the ocean surface to the coveragearea 53. This embodiment also radiates in an upward direction. In thisembodiment, all nodes begin transmission at substantially the same timewith substantially the same phase.

In a variation on this embodiment a VLF sensor 55 is placed on thesurface of the ocean within or near, and preferably at the center of,the coverage area 53, as shown in FIG. 8 . The sensor 55 measures a setof at least one characteristic (e.g. strength) of the VLF signalgenerated by the array of VLF transmitter nodes 50 at a point within ornear, and preferably at the center of, the coverage area 53. Then thesensor 55 transmits, via a radio frequency signal at a noninterferingand different frequency, the relevant characteristics of the measuredVLF signal. This signal is then received by each VLF transmitter node 50by its communication transceiver 22 and used as input to on-boardadaptive beamforming algorithms operating in the controller 14 tocorrect the phasing or timing of a transmission of each VLF transmitternode 50 in the array and may also be used to adjust the position of theVLF transmitter node 50 in the array.

In a related embodiment, the VLF transmitter nodes 50 are arranged intwo planes both parallel to the surface of the ocean with the two planesvertically separated from one another by less than one half wavelengthof the transmission frequency, and preferably separated by approximatelyone quarter wavelength of the VLF transmission. The VLF transmitternodes 50 in each plane may be spaced less than 1 wavelength apart. TheVLF transmitter nodes 50 are preferably phased to radiate straightdownward toward the ocean surface, but not upward away from the oceansurface. In one embodiment, all the VLF transmitter nodes 50 in a planebegin transmission at the same time; however, transmission from the VLFtransmitter nodes 50 in the lower plane is delayed by an amount suchthat radiation from the lower plane adds in phase to the radiation fromthe VLF transmitter nodes 50 in the upper plane and in the direction ofthe ocean. In another embodiment, the transmission from the VLFtransmitter nodes 50 in the lower plane is delayed by an amount thatcancels radiation in an upward direction toward the ionosphere. When theseparation between the two planes is a quarter wavelength of the VLFtransmission frequency, these two embodiments are the same, and the timedelay between the top and bottom plane is given by (vg*(htop-hbottom)),where vg is a group velocity of an electromagnetic wave in air, htop isa height of the upper plane from the ocean surface, and hbottom is aheight of the lower plane from the ocean surface.

In another embodiment, this invention includes an array of electricallysmall VLF transmitter nodes 60 arranged in a line or plane parallel tothe surface of the ocean and supported by satellites 61, as shown inFIG. 9 . In a preferred embodiment, the satellites 61 are arranged inpairs of small satellites 61 connected by a tether 62, with a dipoleantenna placed along the length of the tether. In a preferredembodiment, the tether is conductive and is used as the conductor of thedipole antenna. A variety of satellite and antenna configurations may beused. In this embodiment, antennas 24 for each node in the array areoriented parallel to the surface of the ocean. VLF transmitter nodes 60in the array are spaced roughly one half wavelength from their nearestneighbor and are phased to enhance radiation in the direction of theocean surface and the coverage area 63, while suppressing radiation inother directions. This is accomplished by transmitting from all nodes atthe same time with the same phase.

In a variation on this embodiment a VLF sensor 66 is placed on thesurface of the ocean within and preferably near a center of a desiredcoverage area 63, as shown in FIG. 10 . The sensor 66 measures acharacteristic, e.g. the strength, of the VLF signal generated by thearray of VLF transmitter nodes 60. Then the sensor 66 broadcasts, via aradio frequency signal at a noninterfering and different frequency, thecharacteristic of the measured VLF signal, as described above withreference to FIG. 8 . This broadcasted signal is then received by eachVLF transmitter node 60 by its communication transceiver 22 and used asinput to on-board adaptive beamforming algorithms operating in thecontroller 14 to correct the phasing or timing of a transmission of eachVLF transmitter node 60 in the array and may also be used to adjust theposition of the VLF transmitter node 60 in the array, which may requirerepositioning one or more satellite pairs 61.

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications to the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asdisclosed herein.

The foregoing Detailed Description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations will be apparent topractitioners skilled in the art. No limitation is intended by thedescription of exemplary embodiments which may have included tolerances,feature dimensions, specific operating conditions, engineeringspecifications, or the like, and which may vary between implementationsor with changes to the state of the art, and no limitation should beimplied therefrom. Applicant has made this disclosure with respect tothe current state of the art, but also contemplates advancements andthat adaptations in the future may take into consideration of thoseadvancements, namely in accordance with the then current state of theart. It is intended that the scope of the invention be defined by theClaims as written and equivalents as applicable. Reference to a claimnode in the singular is not intended to mean “one and only one” unlessexplicitly so stated. Moreover, no node, component, nor method orprocess step in this disclosure is intended to be dedicated to thepublic regardless of whether the node, component, or step is explicitlyrecited in the Claims. No claim node herein is to be construed under theprovisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the node isexpressly recited using the phrase “means for . . . ” and no method orprocess step herein is to be construed under those provisions unless thestep, or steps, are expressly recited using the phrase “comprising thestep(s) of . . . ”

What is claimed is:
 1. A system for subsurface transmission comprising:a first array of very low frequency (VLF) transmitter nodes each hostedon an airborne platform, each VLF transmitter node comprising: acontroller; a VLF transmitter coupled to the controller; a data buffercoupled to the controller; a navigation subsystem coupled to thecontroller; and a communications transceiver coupled to the controller;wherein each VLF transmitter node is positioned less than one wavelengthof a desired very low frequency from another VLF transmitter node;wherein data to be transmitted is received by the communicationstransceiver in each VLF transmitter node and stored in the data bufferin each VLF transmitter node; and wherein each respective VLFtransmitter node transmits the data in the data buffer of the respectiveVLF transmitter node with phasing or timing.
 2. The system of claim 1:wherein each VLF transmitter node is positioned roughly a quarterwavelength of the desired very low frequency from another VLFtransmitter node; and wherein transmission for a respective VLFtransmitter node begins at time (t0+vg*(hmax-h)), where h is a height ofa respective VLF transmitter node, hmax is a height of a highest VLFtransmitter node in the array, vg is a group velocity of a Norton wave,and t0 is a time the highest VLF transmitter node begins transmission.3. The system of claim 1: wherein each VLF transmitter node ispositioned roughly a quarter wavelength of the desired very lowfrequency from another VLF transmitter node; and wherein transmission ofthe data in the data buffer for each respective VLF transmitter nodebegins at the same time.
 4. The system of claim 1: wherein the airborneplatforms comprise fixed wing aircraft, rotorcraft, lighter than airvehicles, drones or unmanned airborne vehicles (UAVs).
 5. The system ofclaim 1 further comprising: a very low frequency (VLF) sensor on thesurface of the body of water; wherein the VLF sensor measures acharacteristic of a received VLF signal generated by the array of VLFtransmitter nodes; wherein the sensor transmits the characteristic ofthe received VLF signal; and wherein the transmitted characteristic ofthe received VLF signal is received by the communications transceiverand used by the controller in each VLF transmitter node to adjust atransmission phasing or timing of each VLF transmitter node in the arrayto optimize the characteristic of the received VLF signal.
 6. The systemof claim 5 wherein each VLF transmitter node further comprises: apropulsion system on the airborne platform coupled to the controller;wherein the transmitted characteristic of the received VLF signal isreceived by the communications transceiver and used by the controller ineach VLF transmitter node to adjust a position of the VLF transmitternode in the array.
 7. The system of claim 1 further comprising: a secondarray of very low frequency (VLF) transmitter nodes each hosted on anairborne platform and arranged in a plane parallel to the surface of abody of water, each VLF transmitter node in the second array comprising:a controller; a VLF transmitter coupled to the controller; a data buffercoupled to the controller; a navigation subsystem coupled to thecontroller; and a communications transceiver coupled to the controller;wherein each VLF transmitter node in the second array is positioned lessthan one wavelength of the desired very low frequency from another VLFtransmitter node in the second array; wherein the second array of VLFtransmitter nodes is parallel to the first array of VLF transmitternodes; and wherein the first array and the second array are verticallyseparated from one another by less than one half wavelength of thedesired very low frequency.
 8. The system of claim 7: wherein thetransmission of data from each respective VLF transmitter node in thefirst and the second array is phased or timed.
 9. The system of claim 7:wherein the transmission of data from each respective VLF transmitternode in the first array begins at the same first time; wherein thetransmission of data from each respective VLF transmitter node in thesecond array begins at the same second time; and wherein transmissionfrom a lower plane of the first array is delayed by an amount.
 10. Thesystem of claim 7: wherein the vertical separation between the firstarray and the second array is one quarter wavelength of the desired verylow frequency; and wherein transmission from a lower plane of the firstarray and the second array is delayed by an amount.
 11. The system ofclaim 7: wherein transmission from a lower plane of the first array andthe second array is delayed.
 12. The system of claim 1: wherein each VLFtransmitter node is arranged in a plane that is substantially parallelto a surface of a body of water.
 13. The system of claim 1: wherein eachVLF transmitter node is arranged in a line or plane normal to a surfaceof a body of water.
 14. The system of claim 1: wherein each VLFtransmitter node is positioned less than one half wavelength of thedesired very low frequency from another VLF transmitter node.